Clearly the RFI is being radiated, not just induced into the power or signal cables
Try turning off the installed VHF and disconnecting both power and antenna cable. Now, turn the chartplotter
on and see what you get with a VHF handheld. If you still get interference on Channel 16, the chartplotter itself is at fault and Raymarine
ought to be ashamed. Doubt if you'll get much help from them (or their new owner, FLIR), but it's worth a try to tech support.
If you DON'T get RFI interference, then the problem is likely some weird interaction between the chartplotter and the adjacent VHF. Try temporarily moving the VHF to a distant location, using an extension coax and 12V power cable.